We find that sub-GeV neutrino portal bosons that carry lepton number can condense inside a proto-neutron star (newly born neutron star). These bosons are produced copiously and form a Bose-Einstein condensate for a range of as yet unconstrained coupling strengths to neutrinos. The condensate is a lepton number superfluid with transport properties that differ dramatically from those encountered in ordinary dense baryonic matter. We discuss how this phase could alter the evolution of proto-neutron stars and comment on the implications for neutrino signals and nucleosynthesis.
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